Introduction:

The tissue of the nervous system is very highly membranous and is easily included in a discussion of membrane biochemistry.

A neuron is made up of dendrites, a cell body, and an axon that terminates at a synapse.

Myelinated nerves are wrapped with an insulating layer of membrane derived from Schwann cells.

Myelinated nerves also conduct electricity in a very different manner from unmyelinated nerves; the nerve impulse (electricity) jumps between gaps in the myelin sheath called the nodes of Ranvier.

Electrophysiology:

Recognition of the electrical nature of nerve conduction led physicists and biologists to collaborate in measuring these electrical events.

Membrane has a large electrochemical potential (gradient) across it; the inside is denoted as negative by convention.

outside => 440 mM Na+
20 mM K+
560 mM Cl-
---------------------------------------
--------------------------------------- -60 mV (inside is negative)
inside ==> 400 mM K+
50 mM Na+
50 mM Cl-

Rapid changes in local ion permeability lead to changes in potential that exists across the nerve membrane. The permeability to the movement of ions is regulated by specific membrane proteins, which undergo conformational changes upon an appropriate stimuli.

A nerve action potential can be observed by measuring the change in voltage occurring at one site on an axon (microelectrodes are employed).

The sequence of events is as follows:

a. depolarizing voltage (reaching the threshold value) is attained at a site along the nerve
b. sodium ions flow into the neuron through a protein called the sodium channel
c. a short time later, potassium ions flow out through the same protein

A wave of voltage change moves down the nerve just like an ocean wave comes to the shore. The wave moves but water doesn't; in the case of the nerve the voltage change moves but the bulk of ions don't!

There are two ways to solve the speed of transmission problem

a. increase the diameter of the neuron
b. insulate the nerve (myelin) so that the electrical signal jumps between the gaps (nodes of Ranvier)

Characteristics of nerve conduction

a. action potential does not decay with distance
b. action potential is an all or none (binary - just like a computer) process
c. refractory period, after each impulse passes the nerve is inactive for a short period of time.

The sodium channel:

The sodium channel is a complex, transmembrane protein involved in axonal conduction. As the use of non-ionic detergent for the purification of membrane proteins was developing, biochemists began to purify proteins involved in nerve function.

The sodium channel was detergent solubilized from tissue of the electric organ of the Amazonian electric eel.

Since this protein was not an enzyme, another type of assay was required to permit its purification. A binding assay using radioactive neurotoxin (tetrodotoxin or saxitoxin) was developed. A final requirement for successful purification was the recognition that this protein required the continued presence of lipids for stability.

The purified protein contained two types of subunits, one of molecular weight 260-300 kDa, and another of 30-40 kDa. This unusual protein senses local changes in electrical potential and undergoes a conformation change that open a channel for sodium and potassium ions.

The acetylcholine receptor

involved in synaptic transmission

pentamer --> 2 : 1 : 1 : 1 (54, 56, 58, 60 kDa)

can obtain 2-D arrays --> looks like a doughnut

binding of ACh ---> conformational change --> increased permeability to Na and K

Toxins for ACh receptor

d-tubocurarine
cobratoxin and bungarotoxin
nicotine acts as an agonist (opens) ACh-Receptor

Acetylcholine esterase and nerve gases:

hydrolyzes ACh after its release

                      ACh esterase
Ac-choline   ------------------>  choline  +  acetate

                                                 re-absorbed into vesicles (exchanged for protons)

1. diisopropyl fluorophosphate (DFP)
2. sarin and tabun (military nerve gases)
3. physostigmine
4. parathion